I'm using a Bushmaster M4/A3. If I have the iron sights zero'ed at 50 yards, will it still be zero'ed for 5 yard, 10 yard, 15 yard...well, basically any distance between 1 yard to 25 yards ?

My shooting range will not let me shoot at any lane less than 50 yards (I am restristed to the rifle area, not the handgun area of the range). I use my AR for only as a CQB defense weapon. I do Not move my rear A3 elevation knob, I leave it set to the 6/3 setting (and I don't wanna dork with it).

So, without moving the rear elevation knob, and the gun hitting zero at 50 yards, will I still be zero (plus or minus an inch or less) at distances a lot less than 50 yards ?

There should not be a real change in your zero setting when engaging close in targets. When I shooting targets from point blank to 25 yrds during a carbine class, I did not see any changes on my AR when I had the sights zero at 100yrds.

I just completed an urban rifle course and we were told to zero our rifles at 50 yards so that with our issue ammo the rifle was point of aim point of impact at 50 and 235-250 yards. The trajectory of the .223 round is such that with this zero the poi is no more than 2 inches from poa. From 25 yards out you just put the front sight on target and let 'er rip. At 100 yards you cannot see the 1.3 inches difference between poa/poi. Within 25 yards we are required to hit head shots (2x3.5 inch target area) on a number of drills. At these distances with this small a target it is important to aim about 2 inches higher than you normally would to hit where you want, because your sights are about 2 inches higher than the bore. For center of mass hits the difference is not important.

Another important thing to remember is that the bullet "seeks" the front sight in its trajectory. I couldn't figure out why I was missing to the left consistantly by a couple of inches when firing from Scuba prone or the Gunport positition. I was putting the sight on target and forgetting the round would fly left instead of high.

If you zero at 50 yards the bullet will be 2" low at 10 yards, almost the distance the front sight is above the bore. This is battlesight, not X-ring shooting, you'll have to aim an inch high at 25 yards if you're trying to punch the centers out of playing cards.

By using battlesight you don't have to dick with the sight wheel once it's set right when zeroing.